This is a variable length extension tube with M42x1 threads, one female and one male. This allows extension to be adjustable from 20mm to 30mm. This is useful when precise length of extension is required, such as using some fixed focal length achromat as tube lens, fitting a finite objective onto set of extension tubes.
see the article about this here
NOTICE: All customers will receive 2 emails by default. The order creation notification which tells you that the order has been created, before the payment, and the order status notifications which tells them that the order has been confirmed.